The invention relates to a method of controlling a transmission ratio of a continuously variable toroidal transmission. The continuously variable toroidal transmission has at least one pair of toroidal disks including an input disk and an output disk and at least one intermediate roller in power-transmitting contact with the toroidal disks. The toroidal transmission further has an adjustable pivot-bearing configuration supporting the intermediate roller. The transmission ratio is set by defining a continuously variable pivoted position of the intermediate roller between the input disk and the output disk. The transmission ratio is controlled by feeding an actual transmission ratio back to an input of a control circuit for the transmission ratio on a basis of a control mechanism with reference to a desired transmission ratio by adjustment of a holding force of the pivot-bearing configuration or its adjustment path.
Continuously variable toroidal transmissions transmit power by frictional rolling contact from a toroidal input disk, via an intermediate roller, to a toroidal output disk. The toroidal disks can have a xe2x80x9csemi-toroidal geometryxe2x80x9d or a xe2x80x9cfully toroidal geometryxe2x80x9d.
The at least one intermediate roller between the disks of a pair of toroidal disks is held by an adjustable pivot-bearing configuration. Pivoting the intermediate roller changes its rolling-contact radii on the toroidal disks, leading in turn to a change in the transmission ratio. The normal force required to transmit frictional force is incidentally produced by pressing the toroidal disks axially into contact.
Previous control systems for the transmission ratio are based on a conventional method of controlling the angle of the intermediate roller, e.g. by a proportional-integral-differential (PID) controller. Here, the angle of the intermediate roller is effected by adjusting the steady-state holding force of the pivot-bearing configuration in the transmission that carries the intermediate roller and/or by adjusting its adjustment path. For control, the actual transmission ratio is applied to one input of the control circuit of the control system and reference is made to a desired transmission ratio. In the customary manner for control systems, the transmission ratio is controlled by recourse to the manipulated variable(s) of the steady-state holding force and/or of the adjustment path of the intermediate rollers.
Tests with conventional control systems of this kind have shown that the dynamic control response is in need of improvement. Special transmission configurations have been proposed to this end. Introducing a xe2x80x9chousing anglexe2x80x9d, for example, increases the stability range of the control system, but this applies only in one direction of rotation of the transmission, e.g. when traveling forward.
The technical article by Prof. P. Tenberge titled xe2x80x9cToroidgetriebe mit verbesserten Kennwertenxe2x80x9d [Toroidal Transmissions with Improved Characteristics], VDI Report No. 1393, 1998, pp. 703 to 724 moreover provides an overall view of how the conception and configuration of toroidal traction mechanisms can be optimized in terms of efficiency, control, installation space and power to weight ratio.
It is accordingly an object of the invention to provide a method of controlling the transmission ratio of a continuously variable toroidal transmission which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which controls an angle of the intermediate roller.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of controlling a transmission ratio. The method includes providing a continuously variable toroidal transmission. The toroidal transmission has at least one pair of toroidal disks including an input disk and an output disk and at least one intermediate roller in power-transmitting contact with the toroidal disks. The toroidal transmission further has an adjustable pivot-bearing configuration supporting the intermediate roller. The transmission ratio is set by defining a continuously variable pivoted position of the intermediate roller between the input disk and the output disk. The transmission ratio is controlled by feeding an actual transmission ratio back to an input of a control circuit for defining the transmission ratio and a desired transmission ratio to the control circuit. A deviation between the desired transmission ratio and the actual transmission ratio is compensated for by adjusting a holding force of the pivot-bearing configuration and/or an adjustment path of the pivot-bearing configuration of the intermediate roller. The holding force is pilot-controlled as a function of:
torques acting on the toroidal disks and the pivoted position of the intermediate roller; and/or
the adjustment path of the pivot-bearing configuration of the intermediate roller being fed back one of statically and dynamically into the control circuit.
The object is achieved by adding two additional components to the standard control mechanism, namely, pilot-controlling the holding force as a function of the torques acting on the toroidal disks and of the pivoted position of the intermediate roller and feeding back the adjustment path of the pivot-bearing configuration of the intermediate roller into the control circuit. By these two components, the conventional control concept is significantly extended, enabling various advantages to be achieved. Thus pilot control of the holding force isolates the control circuit from the effect of changes in the torque at the inputs and outputs of the transmission, taking into account the instantaneous pivoting angle of the intermediate roller. Without pilot control of the holding force, the transmission ratio controller merely responds to the effect of changes in torque instead of taking them into account as the cause of disturbances in the desired transmission ratio.
The possibility of feeding back the position of the intermediate roller separately from or in combination with pilot control of the holding force provides an effective system variable for controlling oscillations in the variation of the transmission ratio with time. In particular, it provides an effective method of damping oscillations.
The steady-state holding force Fz is pilot-controlled in accordance with the relation
Fz=M1/R01+M2/R02 
where:
M1 is the torque at the input disk,
M2 is the torque at the output disk,
R01 is the rolling radius of the intermediate roller on the input disk, and
R02 is the rolling radius of the intermediate roller on the output disk.
This is the fundamental relation for a pair of toroidal disks with an intermediate roller.
This relation is expanded to cover a toroidal transmission configuration having a plurality of pairs of toroidal disks and a plurality of intermediate rollers in each toroidal configuration. The steady-state holding force (Fz) is accordingly pilot-controlled as a function of the relation
Fz=(M1/R1+M2/R2)/(NTxc2x7NZ) 
where:
M1 is the torque at the input disk,
M2 is the torque at the output disk,
R1 is the rolling radius of the intermediate roller on the input disk,
R2 is the rolling radius of the intermediate roller on the output disk,
NT is the number of toroidal configurations, and
NZ is the number of intermediate rollers in each toroidal configuration.
In a further mode of the invention, a time derivative (dz/dt) of the adjustment path of the intermediate roller is used as the variable fed back for the purpose of feedback. The fed-back variable can preferably be additionally multiplied by a gain factor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of controlling the transmission ratio of a continuously variable toroidal transmission, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.